Plate count method APHA 2001 for  psychrotrophic fungi in foods

Method of the American Public Health Association

(APHA), as described in 4th Edition of the Compendium of Methods for the Microbiological Examination of Foods (Cousin et al., 2001).

1- Material required for analysis

Preparation of the sample and serial dilutions

•  Diluent: 0.1% Peptone Water (PW) or Butterfield’s Phosphate Buffer

•  Dilution tubes containing 9 ml 0.1% Peptone Water (PW) or Butterfield’s Phosphate Buffer

Enumeration (spread plate)

•  Dichloran Rose Bengal Chloramphenicol (DRBC) Agar plates (for foods with water activity >0.95)

•  Dichloran 18% Glycerol (DG18) Agar plates (for foods with water activity <0.95)

•  Laboratory incubator set to 7±1°C

•  Laboratory incubator set to 17±1°C (optional)

2- Procedure

A general flowchart for the enumeration of psychrorophic fungi in foods using the plate count method APHA 2001 is shown in Figure 1.

The psychrotrophic fungi count by the plate count method APHA 2001 follows the same procedure as that described for the plate count of yeasts and molds (item 7.2), changing the incubation conditions, which may be 7±1°C for 10 days or 17±1°C for 16 hours, followed by 3 more days at 7±1°C.

With regard to the preparation of the samples, homogenizing in a stomacher is more recommended than using a blender, since the latter may favor frag-mentation of the mycelia, which will change the count. If the use of a blender is unavoidable, do no homogenize for more than two minutes. The recommended culture media are  Dicloran Rose Bengal Chloramphenicol agar (DRBC) (for foods in general) and  Dicloran Glycerol 18 (DG-18), for foods with a water activity smaller than 0.95).

Figure 1   Scheme of analysis for enumeration of psychrotrophic fungi in foods using the plate count method APHA 2001 (Cousin et al., 2001).

References

Silva, N.D .; Taniwaki, M.H. ; Junqueira, V.C.A.;  Silveira, N.F.A. , Nasdcimento , M.D.D. and Gomes ,R.A.R .(2013) . Microbiological examination methods of food and water a laboratory Manual. Institute of Food Technology – ITAL, Campinas, SP, Brazil .

Beuchat, L.R. & Cousin, M.A. (2001) Yeasts and molds. In: Downes, F.P. & Ito, K. (eds).  Compendium of Methods for the Microbio-logical Examination of Foods. 4th edition. Washington, American Public Health Association. Chapter 20, pp. 209–215.